A mechanical apparatus performing a motion that imitates a human motion by using an electric or magnetic effect is called a ROBOT. The etymology of ROBOT is said to be originated in Slavic “ROBOTA (a slave machine)”. Although robots have been widely used in Japan since the end of the 1960s, most robots are industrial robots such as manipulators and transport robots used for automated production and non-man production at factories.
Stationary robots such as arm robots installed fixedly at specific sites perform operations such as a parts-assembling operation and a parts-sorting operation only in a fixed and local working space. On the other hand, mobile robots perform operations in an unlimited working space such as acting for a prescribed or unprescribed human operation by moving flexibly along a predetermined route or without a route, and offer a variety of services that substitute for a human, a dog and other animate things. Among others, legged walking robots have advantages in moving up and down stairs and a ladder, walking over an obstacle, and a flexible walk and a walking motion regardless of leveled and unleveled terrain, although these robots are more unstable and more difficult in controlling an attitude and a walk than crawler-type robots and tired mobile robots.
Recently, research and development of legged walking robots such as human-shaped robots, i.e., humanoid robots designed by modeling after a body mechanism and a motion of biped walking animals such as a human have advanced, and thus expectations for practical use have been increasingly built up. For example, a biped-walking, humanoid robot “SDR-3X” was disclosed by Sony Corporation on Nov. 25, 2000.
The following two are exemplary viewpoints from which the importance of research and development of a biped walking robot called a human-shaped robot, i.e., a humanoid robot is understood.
One is a viewpoint from human science. More particularly, fabricating a robot having a structure imitating human lower limbs and/or upper limbs and devising the control method therefor lead to technologically solving a mechanism of natural human motions including walking through a simulation process of the human motions. Such study is expected to significantly contribute to promoting a variety of other research fields for human athletic mechanisms such as ergonomics, rehabilitation technology, and sports science.
The other is a viewpoint from the development of practical robots for supporting living activities as a partner of man, that is, for supporting human activities in various daily environments including a living environment. Such kinds of robots are required to learn the way of adapting to people, each having different personalities, or to different environments while being taught by the people, and to further develop the functions thereof in various aspects of human living environments. A human-shaped robot or a robot having the same shape or the same structure with man is expected to function effectively for smooth communication with man.
For example, when teaching a robot the way of passing through a room on site while avoiding an obstacle on which the robot must not step, an operator expects to more easily teach the above-mentioned way to a biped walking robot having a similar shape with that of the operator than to a crawler-type robot or a quadruped robot having a structure totally different from that of the operator. Also, it must be the easy way for the robot to be taught (for example, refer to Takanishi: Control of Biped walking robot, Society of Automotive Engineers of Japan, Kanto Charter, <KOSO> No. 25, Apr. 1996).
Most of a human working and living spaces are formed in accordance with a body mechanism and patterns of behavior of a human who walks erect with two legs. In other words, so many obstacles exist against a current mechanical system having wheels and a driving unit as moving means to move in the human living space. In order for the mechanical system, i.e., the robot, to offer services that support or substitute a variety of human operations and to be thus deeply involved in the human living space, it is preferable that the robot have a movable range substantially the same as that of a human. This is the reason why the legged walking robot is greatly expected to come into practical use. It can be said that the robot must have a human structure so as to improve affinity to the human living environment.
A large number of attitude controls and stable walk technologies about biped walking robots have been proposed. The stable walk as mentioned above is defined as a legged locomotion without falling down.
An attitude stabilization control of a robot is extremely important for preventing the robot from falling down, because falling-down leads to suspending the performing operation of the robot and also requires a considerable amount of energy and time for standing up and restarting the operation from falling down. Most importantly, falling-down of the robot causes a risk of a fatal damage to the robot itself or to an opposing obstacle colliding with the falling robot. Accordingly, the attitude stabilization control for walking and other leg-moving operations is the most important technical matter in designing and developing a legged walking robot.
While walking, a gravitational force, an inertia force, and a moment due to these forces from a walking system act on a road surface because of a gravity and an acceleration caused by a walking motion. According to a so-called D'Alambert principle, these forces and the moment balance a floor reaction force and moment as a reaction from the road surface to a walking system. As a result of the dynamic deduction, a supporting polygon formed by the grounding points of foot bottoms and the road surface has a point at which pitch-axis and roll-axis moments are zero, in other words, a zero moment point (ZMP) on the sides of or inside the supporting polygon.
Most proposals about the attitude stabilization controls and falling prevention of legged walking robots use this ZMP as a criterion for determining walking stability. Generation of a biped walking pattern based on the ZMP criterion has advantages in easily considering a toe condition of kinematical constraints in accordance with a road surface profile and the like since the grounding point of the foot bottom can be preset. Using ZMP as a criterion for determining stability means that a trajectory instead of a force is used as a target value for a motion control, thereby increasing the technical feasibility of the robot.
A general idea of ZMP and an application of ZMP to a criterion for determining stability of a walking robot are described in “LEGGED LOCOMOTION ROBOTS” written by Miomir Vukobratovic (“Walking Robot and Artificial Leg” written by Ichiro Kato, et al., The Nikkan Kogyo Shimbun Ltd.).
In general, a biped walking robot such as a humanoid robot has a center of gravity at a higher position and a narrower ZMP region for stable walking than a quadruped walking robot. Therefore, such an issue of an attitude variation depending on a road surface profile is especially important to the biped walking robot.
However, the legged walking robot has just taken a first step from a development stage to a practical application stage and a large number of technological problems still remain unsolved.
The legged walking robot expected to take an active role in the human living environment basically has a plurality of “limbs” formed of rotating joints and is required to perform a high-speed switching operation between a closed link mode and an open link mode with the outside world or a working object so as to achieve stable biped walking, a stable two-arms operation, or the like.
For example, the legged walking robot performs a variety of leg-moving operations such as walking by usually standing alternately with a single or both of right and left movable legs. When the legged walking robot changes its link mechanism with a floor, a wall, or the like from an open link mechanism to a closed link mechanism, for example, when it switches its standing mode from a single-leg standing mode to a both-legs standing mode, it often has a difference between an expected value from its control system and an actual value, for example, at its grounding toe.
Such a difference between the expected value and the actual value causes “detachment” which means that the toe has not yet grounded at the time when it is expected to ground or “crash” which means that the toe has grounded earlier than expected. These detachment and crash have a great effect on an attitude stabilization control of the body of the legged walking robot.
Heretofore, there have been attempts for achieving a high-speed switching operation from the open link mode to the closed link mode with a feedback control by using software on the basis of a force signal from a force sensor disposed at the top of a limb or a torque signal from an actuator for driving a joint. However, achieving a stable motion with this method is very difficult from a technical viewpoint since a short feedback cycle, a high resolution, a high speed, and a large acceleration for driving the joint are required to an unrealistic degree.